The use of air as a coolant is preferable from the point of view of environmental protection and safety, but does not provide sufficient properties in terms of energy efficiency in comparison with the case where chlorofluorocarbons, ammonium gas or the like is used. In the case where air is used as a coolant in a facility where it can be directly blown into, such as refrigerated warehouses, however, there is a possibility that the total cost can be lowered to that of the existing system by devising a means for omitting fans within a warehouse or defrosting systems. At present, the use of chlorofluorocarbons as a coolant has already been regulated from an environmental point of view, and it is also desired to avoid the use of other gases as a coolant as much as possible. Therefore, air cycle refrigerating/cooling systems in which air is used as a coolant in such applications as described above have been proposed (for example, Patent Document 1 and Non-Patent Document 1).
In addition, it has been stated that the theoretical efficiency of cooling with air becomes the same as or higher than that with chlorofluorocarbons or ammonium gas in a deeply cold range from −30° C. to −60° C. (Non-Patent Document 1). It has also been stated, however, that the above described theoretical efficiency of cooling with air can be attained only with peripheral apparatuses which are optimally designed. The peripheral apparatuses include a compressor, an expansion turbine and the like.
Turbine units where a compressor rotor and an expansion turbine rotor are attached to the same main shaft are used as the compressor and the expansion turbine (Patent Document 1, Non-Patent Document 1).
Here, magnetic bearing type turbine compressors where a turbine rotor is attached to one end of the main shaft, a compressor rotor is attached to the other end, and the above described main shaft is supported by a journal bearing and a thrust bearing, each bearing is controlled with a current through an electromagnet, have been proposed as a turbine compressor for processing a process gas (Patent Document 2).
In addition, a reduction in the thrust load which affects the rolling contact bearing through the use of a thrust magnetic bearing has been proposed for gas turbine engines in order to prevent the thrust load which affects the rolling contact bearing for supporting the main shaft from making the life of the bearing shorter (Patent Document 3).    Patent Document 1: Japanese Patent No. 2623202    Patent Document 2: Japanese Laid-open Patent Publication No. 7-91760            Patent Document 3: Japanese Laid-open Patent Publication No. 8-261237        Non-Patent Document 1: Nikkei Mechanical Magazine, “Cooling Air with Air,” issued on Nov. 13, 1995, No. 467, pages 46 to 52        
As described above, air cycle refrigerating/cooling systems require an optimally designed compressor and expansion turbine in order to attain the theoretical efficiency of air cooling, which becomes of a high efficiency in the deeply cold range.
As described above, turbine units where a compressor rotor and an expansion turbine rotor are attached to the same main shaft are used as the compressor and the expansion turbine. In these turbine units, the compressor rotor can be driven with the power generated by the expansion turbine, and thus, the efficiency of the air cycling refrigerator is increased.
In order to attain the efficiency for practical use, however, it is necessary to keep the gap between the rotors and the housing microscopic. Fluctuations of this gap prevent a stable high speed rotation, and thus, cause a reduction in the efficiency.
In addition, air which affects the compressor rotor and the turbine rotor makes the thrust force affect the main shaft, and thus, a thrust load is imposed on the bearing for supporting the main shaft. The rotational speed of the main shaft of the turbine unit for refrigerating/cooling air cycle is 80,000 to 100,000 rotations per minute, which is a very high speed in comparison with bearings for general applications. Therefore, thrust loads as described above cause the durability of the bearing for supporting the main shaft to decrease and the life to become shorter, and thus, decreases the reliability of the turbine unit for air cycle refrigerating/cooling. It is difficult to put a turbine unit for air cycle refrigerating/cooling into practice, and thus, put an air cycle refrigerating/cooling system into practice without solving the problem of the durability of the bearing as described above. The above described technologies disclosed in Patent Document 1 and Non-Patent Document 1, however, do not solve the problem where the durability of the bearing against the thrust load for such a high speed rotation decreases.
In such a magnetic bearing type turbine compressor as that of Patent Document 2 where the main shaft is supported by a journal bearing made up of a magnetic bearing and a thrust bearing made up of a magnetic bearing, the journal bearing does not have a function of restricting movement in the axial direction. Therefore, when there is an unstable factor in the control of the thrust bearing, it is difficult to provide a stable high speed rotation while keeping a microscopic gap between the above described rotors and housing. In the case of a magnetic bearing, there is also a problem of contact at the time of power failure.
In, addition, in the turbine unite for refrigerating/cooling air cycle of the case where the main shaft is rotatably supported by a rolling contact bearing and part or the entirety of the thrust force applied to the main shaft is supported by an electromagnet on the basis of the output of a sensor for detecting the thrust force which affects the main shaft, when the precision in detecting the thrust force which affects this main shaft is low, the thrust force applied to the bearing cannot be effectively reduced, and thus, the durability of the bearing cannot be secured.